Tetra air pump rated for 100 gallons. Cheap gang valve. 4x cheap sponge filters running 3x live food cultures (Daphnia magna, Moina macrocopa, Gammarus sp.) and 1x indoor pond. 3x Swiss Tropicals Hamberg Mattenfilter (NOT cheap, but GREAT value)

I really love this setup. I'm setting up a lot of expansion in my home office right now and it will be 100% air-powered filters throughout.